Global and Regional Vigilance: Are There Two Types of Local Sleep?

PURPOSE: Since the discovery of uni-hemispheric sleep in dolphins we have accepted that sleep does not need to be a global cortical state. The term "local sleep" has been introduced for this phenomenon. FINDING: However, the term seems to be used for two distinct types of local sleep. The first type, which is seen in dolphins, is the one where two different vigilance states are found simultaneously on the cortex like in non-rapid eye-movement (NREM) sleep parasomnias in humans. In the second type, only one vigilance state is observed, but the activity of local neurons is changed by changing their workload during previous waking. There are not only functional, but probably also mechanistic differences between the two types of local sleep, where the first originates from deeper brain areas, and the second is induced locally on the cortex. CONCLUSION: Recognizing this distinction helps to ask clearer defined questions when investigating local sleep.